An engine disposed in a vehicle includes an exhaust gas recirculation system (EGR system) for returning EGR gas to its air intake side. The EGR system includes an EGR passage for guiding EGR gas from an exhaust passage of the engine to an intake passage; an EGR valve that regulates the amount of EGR gas in this EGR passage returned to the air intake side through the regulation of its opening degree; and an engine control unit (ECU) that performs control of the opening degree of this EGR valve (specifically, control of energization of an electric actuator disposed in the EGR valve).
A main feature of an EGR valve of a conventional technology will be described in reference to FIG. 4. In an EGR valve 101, a butterfly valve 106 (valving element having generally a shape of a disk) is arranged to be inclined relative to a shaft 110, and a seal ring 108 which prevents leakage at the time of valve closing is provided at an outer peripheral edge of the butterfly valve 106. The inside of a gas passage 102 is under a severe environment where the EGR gas flows and strongly-acid condensed water is thereby produced. For this reason, the seal ring 108 is formed from metal such as stainless steel which is excellent in acid resistance.
If a member in contact with the seal ring 108 is an aluminum material, from which a valve body 103 is made, a position of the valve body 103 in contact with the seal ring 108 is immediately corroded and worn out. Accordingly, in the previously proposed EGR valve 101, a bore ring (cylindrical nozzle) 9 made of stainless steel is disposed at the region in contact with the seal ring 108 so as to limit corrosion and wear of the region in sliding contact with the seal ring 108 (see, for example, JP-A-2007-285311).
In this manner, in the conventional EGR valve 101, every time the butterfly valve 106 is opened and closed, the seal ring 108 made of stainless steel and the bore ring 109 made of stainless steel are rubbed against each other. However, although the rubbing of stainless steels on each other limits their wear rather than rubbing between stainless steel and aluminum, two stainless steels are not a combination excellent in wear resistance.
Therefore, in the previously proposed EGR valve 101 in which the seal ring 108 made of stainless steel and the bore ring 109 made of stainless steel are rubbed against each other, there is concern that wear is caused due to its long-term use and EGR gas may leak from the worn-out region. If EGR gas leaks out of the worn-out region at the time of closing the EGR valve 101, the specified amount of fresh air (amount of air required by an ECU) cannot be ensured. As a result, there is concern about a defect such as reduction of engine output or an engine failure.